The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft is driven by several turbojet engines each housed within a nacelle. A nacelle has generally a tubular structure along a longitudinal axis comprising a fixed upstream section constituted by an air inlet upstream of the turbojet engine, a fixed median section intended to surround a fan of the turbojet engine, a downstream section accommodating thrust reversal means and intended to surround the combustion chamber of the turbojet engine, the upstream and the downstream of the nacelle being defined with reference to the flow direction of the airflow in the nacelle in direct jet operation, the upstream of the nacelle corresponding to a portion of the nacelle by which the airflow enters, and the downstream corresponding to an ejection area of said airflow.
A propulsion unit for an aircraft is constituted by a nacelle and by a turbojet engine. A downstream portion of a propulsion unit 1 for an aircraft is shown schematically in FIG. 1, in the closed position. Such a propulsion unit 1 is typically suspended from a fixed structure of the aircraft, for example under a wing or on the fuselage, by means of a suspension mast 3 fastened to the turbojet engine 5.
The modern nacelles are intended to accommodate a bypass turbojet engine capable of generating, by means of the rotating fan blades, a hot airflow (also called “primary flow”) coming from the combustion chamber of the turbojet engine, and a cold airflow (“secondary flow) which circulates outside the turbojet engine through an annular passage, also called flow path. The terms “downstream” and “upstream” refer here to the flow direction of air circulation in the turbojet engine. A turbojet engine includes usually a section called “upstream” section comprising the fan blades and a section called “downstream” section accommodating the gas generator.
The downstream portion of a propulsion unit comprises a thrust reverser 7 comprising two semi-annular half-cowls 9 surrounding the downstream section of the turbojet engine 5.
The thrust reverser 7 shown in FIG. 1 is of the “D-duct” type, that is to say, each half-cowl 9 of the thrust reverser comprises an outer fixed half-structure 11, called “Outer Fixed Structure” (OFS) and a concentric inner fixed half-structure 13, called “Inner Fixed Structure” (IFS), surrounding the structure itself of the engine downstream of the turbojet engine and secured to the outer fixed half-structure 11.
The inner 11 and outer 13 half-structures define a flow path 15 intended to channel the flow of cold air which circulates outside the turbojet engine. The primary and secondary flows are ejected from the engine by the rear of the nacelle.
The outer fixed structure 13 comprises an upstream portion and a downstream portion on which is mounted a cowl (not shown in Figures) which is movable in translation and adapted to cover thrust reversal means when the latter are not used. In case there is a desire for the thrust reversal means, the movable cowl is translated downstream to a position called “reverse jet position” in which the thrust reversal means are discovered.
Each half-cowl 9 of the thrust reverser is mounted on a hinge 17 fixed to the suspension mast 3. In order to carry out maintenance of the turbojet engine 5, it is known to access the suspension mast 3 by pivoting each half-cowl 9 of the thrust reverser about axes 19 substantially collinear with a longitudinal axis 21 of the propulsion unit, thanks to the hinges 17, as shown in FIG. 2 schematically illustrating the downstream portion of the propulsion unit 1 in the maintenance position. In this position, each half-cowl 9 of the thrust reverser has pivoted about the axes 19, and an operator can access the turbojet 5 to perform maintenance of the turbojet engine.
Also known from the prior art are thrust reversers of the C-duct type, for which each half-cowl comprises an outer half-structure and an inner half-structure, similar to those described with reference to FIGS. 1 to 4. Unlike the D-duct type thrust reverser described above, the inner half-structure of a half-cowl of D-duct type thrust reverser is not secured to the outer half-structure of said half-cowl of thrust reverser. Whatever the type of thrust reverser, C-duct or D-duct, the passage from an operating position according to which the thrust reverser is closed to a maintenance position according to which the thrust reverser is opened, is typically carried out by the actuation of a cylinder.
Reference is made to FIGS. 3 and 4 which illustrate the downstream portion of the propulsion unit 1 in an isometric view, the thrust reverser 7 being shown respectively in the operating position and in the maintenance position.
The pivoting of each half-cowl 9 of the thrust reverser 7 is obtained by the actuation of a cylinder 23 called “COS” cylinder, acronym frequently used for “Cowl Opening System.” The cylinder 23, which can be, for example, hydraulic, electric or pneumatic, comprises a fixed portion mounted on a fitting 25 mounted on the turbojet engine 5 and a movable portion fixed on a fitting 27 mounted on the half-cowl 9 of thrust reverser.
The holding of the cowls in the maintenance position is further reinforced thanks to a safety connecting rod 29 frequently designated by the acronym “HOR” for “Hold Open Rod.” The connecting rod 29 comprises a first end fixed to the fitting 25 mounted on the turbojet engine 5, and a second end fixed to a fitting 31 mounted on the half-cowl 9.
Access to the turbojet engine can also involve the opening of half-cowls constituting the fan casing of the turbojet engine. The opening of these half-cowls can also be performed in the manner of what has been described with reference to FIGS. 1 and 2, that is to say by pivoting these half-cowls about axes substantially collinear with the longitudinal axis of the propulsion unit by means of cylinders COS and connecting rods HOR.
Among the maintenance operations currently carried out on a turbojet engine, there are maintenance operations called “regular” maintenance operations and maintenance operations called “irregular” maintenance operations.
The regular maintenance operations concern equipment of the propulsion unit which involve frequent control, typically several times a month. It is about equipment generally positioned close to the gas generator of the turbojet engine, more generally in an area of the propulsion unit having an opening angle of each half-cowl of thrust reverser comprised between about 30 and 45 degrees.
The irregular maintenance operations in turn concern equipments of the propulsion unit for which an infrequent control is involved, typically once or twice during the life of the aircraft. It is about equipment of the propulsion unit generally positioned close to the suspension mast of the turbojet engine, more generally in an area of the propulsion unit having an opening angle of each half-cowl of thrust reverser comprised between about 50 and 60 degrees.
The regular or irregular maintenance operations are accomplished thanks to the cylinder COS 23 and the connecting rod HOR 29 described with reference to FIGS. 3 and 4.
For this purpose, the cylinders 23 of the propulsion unit have a stroke enabling reaching the opening angles of the half-cowls 9 for carrying out irregular maintenance operations. The connecting rods 29 and the fittings 25, 27 and 31 are in turn sized to support the forces generated by the opening of the half-cowls during a passage in the irregular maintenance position. Such strokes of the cylinders, such a sizing of the fittings and of the connecting rods increase substantially the mass of the nacelle.
Furthermore, these cylinders COS and these connecting rods HOR of the prior art have two distinct deployment lengths, each length allowing to obtain an opening angle for regular maintenance operations and for irregular maintenance operations. Each of these two positions involve the presence of a stabilizer and of a stability indicator of the connecting rods and cylinders, aiming at indicating to the operator the reached position and inhibiting any unexpected closing of the cowl. The presence of these stabilizer and indicator of position on the cylinders COS and connecting rods HOR of the prior art complicates and increases the mass of these connecting rods and cylinders. Further, in addition to the limited reliability offered by these connecting rods and cylinders of the prior art, their manufacturing complexity leads to relatively higher manufacturing costs.
According to another form of the prior art, two attachment points of the connecting rod HOR and two attachment points of the cylinder COS equip the cowl, enabling reaching the regular and irregular maintenance positions with connecting rods and cylinders having a single deployment length. The cowl comprises for this purpose fittings adapted to support the cylinders COS and connecting rods HOR adapted to support the two maintenance positions. The presence of these two fittings for each cylinder and for each connecting rod increases the mass of the nacelle.